The present invention relates to a capillary for bonding copper wires between a semiconductor circuit chip and a corresponding terminal connector of a semiconductor device.
Electrical bonds are provided between a semiconductor chip and corresponding pins or leads on a metallic leadframe of a semiconductor device. The bonding pins are formed monolithically with the leadframe and allow the semiconductor circuit to be mounted and connected to a preprinted electronic board.
Preset contact pads are arranged on the peripheral region of the semiconductor chip, and are bonded to corresponding pins or leads of the semiconductor device. Fine wires are used for this purpose between the pads and the inward ends of the corresponding pins or leads. The electrical bonds are currently provided using gold and aluminum wires because of their reliability and yield.
However, the possibility of using copper wires instead of gold and aluminum wires has recently been verified. The physical properties of copper make it particularly advantageous with respect to gold and aluminum wires. These properties can be summarized as low electrical resistance, phase stability and reliability when the copper wire is arranged on the silicon pads of the semiconductor chip, low cost and a higher melting current than wires of the same diameter made of different materials.
The use of copper wires is hindered by two main drawbacks. A first drawback is the activation problem related to the formation of loose balls. A ball is a copper sphere that is provided to mutually bond the copper wire and the silicon pad onto which the copper wire is to be bonded. The copper sphere is obtained by melting the copper wire, for example, by applying a suitable voltage between the wire and an electrode with a consequent electrical discharge. In these conditions, the copper is subject to oxidation, and this entails the generation of a loose irregular ball and the deterioration of the soldering process.
A second drawback is that the copper wire is very hard, and this can lead to cracking of the silicon pad of the chip. The first drawback related to the formation of the loose ball or sphere has been solved by using a forming gas. The cracking problem has instead been eliminated by a correct choice of the parameters for bonding the copper wire to the semiconductor chip pad.
However, the presence of an irregularly shaped copper ball is a drawback which the current art has not yet solved. The irregular shape of the copper ball is formed at the end of the copper wire, and therefore allows bonding of the wire to the silicon pad of the semiconductor chip. This irregular shape is mainly due to the geometric shape of the capillary in which the copper wire is passed and which is used to form the copper ball, and most of all, to bond it to the silicon pad of the semiconductor chip.
The optimum configuration to be obtained for the copper ball is one in which the diametrical axis of the ball coincides with the axis of the copper wire bonded thereto. Situations in which the copper ball is substantially tangent to the outer surface of the copper wire to which it is bonded are generally not ideal for bonding, such as ultrasonic bonding of the copper ball to the surface of the semiconductor chip (silicon pad), for example.
FIG. 1 is a sectional view of the end portion of a conventional capillary in which a copper wire is inserted with a copper ball at its lower end. The capillary is designated by the reference numeral 1, the copper wire is designated by the reference numeral 2 and the ball is designated by the reference numeral 3.
In this case, the copper wire 2 is perfectly centered within the hole 4 of the capillary 1, and the result of bonding the ball to the surface of a pad 5 of a semiconductor chip is shown in FIG. 2, which clearly shows that the capillary 1 applies a modeling action to the ball 3, which assumes a shape such as the one shown in detail in FIG. 2a, with a substantially flat annular peripheral region 6.
FIG. 3 instead illustrates a situation in which the capillary 1 is used to solder the copper wire 2 but the wire is not centered within the hole 4 of the capillary 1, and therefore the copper ball 3 is not shaped as in FIG. 2a, i.e., symmetrical. FIG. 3a clearly illustrates how the substantially flat annular peripheral region 6 in this case is not symmetrically formed with respect to a diametrical axis of the copper ball 3. A deformed ball 3, as shown in FIG. 3a, would not be considered acceptable for the purpose of bonding a lead to the silicon pad of a semiconductor chip.
An object of the present invention is to provide a capillary for the electrical bonding of a copper wire between a semiconductor chip and the corresponding pins of a semiconductor device which accommodates the chip, wherein the deformation of the copper ball is such as to produce an annular peripheral region which is substantially flat and substantially symmetrical with respect to a diametrical axis of the copper ball.
Another object of the present invention is to provide a capillary in which the shape of the end that is adapted to make contact with the copper ball is such as to achieve, on the copper ball, an annular peripheral region that is substantially flat and symmetrical with respect to a diametrical axis of the copper ball.
Yet another object of the present invention is to provide a capillary which offers adequate characteristics of yield in the process for bonding the copper wire to the semiconductor chip.
Another object of the present invention is to provide a capillary which is highly reliable, relatively straightforward to manufacture and at competitive costs.
These and other objects in accordance with the present invention are achieved by a capillary for electrical bonding between a semiconductor chip and corresponding pins of a semiconductor device in which the chip is accommodated using a capillary comprising a body whose terminal portion is substantially frustum shaped and has a diametrical through hole which allows the passage of a copper wire for electrical bonding between the chip and the semiconductor device. The portion of the body that is adjacent to the lower end of the through hole is flared, with a flaring diameter and a flaring angle which allows formation of a substantially flat annular peripheral region on a copper ball when the copper ball placed at the lower end of the copper wire is deformed by the action of the capillary. The formation of the substantially flat annular peripheral region is independent of the position of the copper wire within the through hole of the body of the capillary.